The results of under-pressurization and over-pressurization of natural gas in customer's piping networks have been well documented: (i) Under-pressurization leads to unsafe use of the user's appliances such as production of increased amounts of formaldehyde, nitrous oxides, carbon monoxide and soot; (ii) Over pressurization can cause explosions, well documented by gas explosions in Alemeda County, Calif. in the 1990's.
Federal pipeline safety regulations have been proposed that require installations of excess flow valves in mitigating problems associated with over-pressurization of the gas source. However, experience has now shown that such valves do not operate properly due primarily to the problem of contaminant build-up at valve seats and valve plunger-liner interfaces. It is believed such build-up results from aperiodic loading of gas delivery network in which contaminants become clumped into packets due to dampening effects of the compressor driven network and multiple customer outlet usage that adds the aforementioned aperiodic loading within the network. Furthermore, in some circumstances noted in association with detection of under-pressurization, both the closure valve and its detection circuitry can be adversely affected by the presence of such contaminants.
In my application Ser. No. 155,957 for "DETECTION OF NATURAL GAS WITHIN A CUSTOMER'S DOMAIN" filed Nov. 19, 1993, now U.S. Pat. No. 5,437,180, hereby incorporated by reference, I teach that the temperature of the gas stream entering the end user's internal piping system has been found to be fairly stable (at about 50 degree F.) while the temperature of the end user's internal piping system (irrespective of total length) has been found to vary as a function of the ambient temperature exterior of the end user's internal piping system. Each of such temperatures can thus be used in association with a differential pressure sensing and conditioning circuit that uses a piezoresistive pressure sensor which measures differential pressures between the natural gas and ambient air entering the transducer to provide a pair of output signals, wherein the conditioned difference between these signals is related to the magnitude of flow of the natural gas based on sensed temperature changes at measured by first and second thermistors.
In such a sensor, there is provided four piezoresistive elements in or on a flexible diaphragm. Two ports are provided to provide ambient air and natural gas samples. When the diaphragm is flexed because of differential pressure, a stress is placed on the piezoresistive elements, causing them to change resistance. The resistance of such elements at any one time, is an accurate indication of differential pressure. But since both the resistance of the piezoresistive elements and its sensitivity to change as a function of stress, are dependent on a clean interface for natural gas passage, contamination due to impurities in the natural gas can unduly affect accuracy of results.
While my circuit can generate a correction voltage due to environmental changes such as temperature to adjust the differential signal, calibration may be difficult as a function of time when contamination build-up occurs due to aperiodic loading.
In the adjustment circuit set forth above, the sensor signal is adjusted to provide a desired output signal using the temperatures of the gas stream and the metal mass of the piping system of the end-user, such temperatures being introduced to the adjustment circuit via the first and second thermistors. Such output signal has also been found to be surprisingly indicative of small flow of natural gas from the gas network being monitored if the natural gas stream remains free of contamination. The final process signal is then further conditioned and used to drive the inverting input of an operational amplifier operating as a conventional comparator, such amplifier having its non-inverting input connected to ground through a voltage divider to establish a set point level whereby when the inverting signal is below the set point level, the output of amplifier goes HI to drive a visual, audio or other type of alarm circuit to alert the end-user of leakage flow of natural gas within his piping system. Such alarm circuits are conventional in the art.
In the present invention, instead of driving a single alarm circuit, a pair of differential pressure sensing and conditioning circuits each using a piezoresistive pressure sensor, are placed adjacent to each other along the piping network downstream of a filter assembly and conditioned to be alternatively activated as function of occurrence: (i) an under-pressurization condition leading to unsafe use of the user's appliances such as production of increased amounts of formaldehyde, nitrous oxides, carbon monoxide and soot; (ii) Over-pressurization condition that can cause explosions within the end user-customer's domain. Such activation can be automatically utilized such as to activate an alarm circuit to warn of the condition.
In more detail, within one of the circuits, the set point level is set at a low level where when the inverting signal is below such setpoint level, the output of amplifier goes HI to drive say, an associated alarm circuit. In the other circuit, the set point level is set at a high level where when the inverting signal is above the setpoint level, the output of the amplifier goes HI to drive, say the same associated alarm circuit.
Accordingly, the present invention provides an highly accurate detection method for both under pressurization and over pressurization of the natural gas as detected within the gas piping network owned by the customer, such system or network being found between the gas meter maintained and owned by the natural gas supplier and the appliances owned and operated by the gas customer. Such method takes into account temperature variations within the gas stream and exterior of the end user's internal piping system and provides for the absence of contaminants to inhibit detector operation due to the presence of the filter assembly between the gas meter owned by the gas supplier and the detection system of the invention. Note that such detection method occurs after the gas stream has been filtered to remove all substances that pose a threat to precise detector operation. That is, the detection system of the invention is downstream of the conventional gas meter owned by the gas supplier and a gas filtering system as previously taught in the patents cited above, such detection system providing an active indication of under and over-pressure conditions within the end user-customer's piping system or network whereby the set point level of the pair of comparator circuit is easily established for each such contition.